Properly implement Debug for Window and EventLoop types (#3297)

For EventLoop, EventLoopBuilder, EventLoopProxy and by requiring it as
a supertrait of Window and ActiveEventLoop.

It is especially useful for user to be able to know that Window is Debug.

src/platform_impl/apple/appkit/window.rs

@@ -16,6 +16,7 @@ use crate::window::{
     WindowAttributes, WindowButtons, WindowId, WindowLevel,
 };
 
+#[derive(Debug)]
 pub(crate) struct Window {
     window: MainThreadBound<Retained<NSWindow>>,
     /// The window only keeps a weak reference to this, so we must keep it around here.